Spacecraft, satellites, or other vehicles in orbit experience a number of factors such as aerodynamic drag that can cause undesirable changes in attitude. Attitude control systems (ACSs) are often utilized to control/adjust the attitude of a spacecraft, satellite, or other vehicle. Such ACSs can include various rotating inertia members such as reaction wheel assemblies (RWAs), control momentum gyroscopes (CMGs) and similar actuators.
A RWA is a type of attitude control device that can be used in attitude control systems to exchange angular momentum with a space vehicle. A reaction wheel assembly typically includes a very large and heavy flywheel that is fixed in a body frame or housing. An electric motor is used to produce a torque along a spin axis of the flywheel so that the flywheel rotates to produce a force that opposes motion in one plane. The electric motor and wheel are supported on a rotor that acts like an axle. The rotor is positioned between bearings located at opposing ends of the rotor so that the rotor is allowed to spin within bearings.
A CMG is another type of attitude control device that can be used in attitude control systems. A CMG usually includes a spinning rotor (e.g., flywheel) and one or more motorized gimbals that tilt the rotor's angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque that rotates the spacecraft. The spin axis of the CMG can be changed by moving the rotor using the gimbal assembly. The torque produced is orthogonal to the spin axis and the gimbal axis. CMGs differ from RWAs in that the latter applies torque simply by changing rotor spin speed, but the former tilts the rotor's spin axis without necessarily changing its spin speed. In general, CMGs are more power efficient.
During launching and/or ascent of a spacecraft vibrations and/or harmonic forces are generated that result in loads that are distributed throughout the load-bearing structure of the spacecraft and its subsystems and components. A portion of these forces are imparted at the bearings of the RWA or CMG, and if the forces exceed the levels that the bearings were designed to accommodate, the bearings could be overstressed.
As the size of the rotor used in a RWA/CMG increases, loading on the bearings during launching also increases. It would be desirable to reduce the loads on the bearings even though rotor size has increased.
To handle the increased forces and torques on the bearings, some RWAs/CMGs simply increase the size of the bearings. However, this is not an option or is undesirable in many RWAs/CMGs. The use of bulkier, heavier bearings not only increases the mass/size of the RWA/CMG, it also increases drag torque on the shaft of a RWA/CMG, which can increase power requirements and decrease the life of the RWA/CMG. As such, it is often desirable to use smaller bearings since they generally have lower friction drag. Smaller bearings can also increase the life of the bearings.
When smaller bearings are used in RWAs/CMGs, a mechanism is needed to ensure that forces and torques on the bearings do not exceed their operating capabilities. As such, there is a need to reduce the loading on the smaller bearings to acceptable levels while maintaining high margins and reliability.